Pressure increase at the PV valve inlet above the set pressure, when the PV valve is relieving. NOTE 1 Overpressure is expressed in pressure units or as a percentage of the set pressure. NOTE 2 The value or magnitude of the overpressure is equal to the value or magnitude of the accumulation when the valve is set at the maximum allowable working pressure or design pressure and the inlet piping losses are zero.
Source: API STD 2000, Venting Atmospheric and Low-pressure Storage Tanks, Sixth Edition, November 2009. Global Standards